Sugar yield,nitrogen uptake by sugar beet and optimal nitrogen fertilization in relation to nitrogen soil analyses and several additional factors

To test the relative usefulness of different methods of chemical analysis for soil nitrogen fractions in the assessment of the fertilizer nitrogen needs of sugar beet, different doses of nitrogen were applied in field experiments laid out during the years 1985–1991. The chemical methods used were N mineral (NO_–³+NH₊⁴) analyses on soil samples taken in late winter, and extraction with 0.01 M CaCl₂ from soil samples taken the preceding autumn and in late winter. The results of the chemical methods were evaluated in models using estimated optimum nitrogen fertilization, nitrogen present in beets or beets+leaves at leaf maximum and sugar yield as variables. In addition, parameters such as estimates of possible rooting depth and mineralization capacity of the soil were also included in the model. All models for estimating nitrogen fertilization need showed low R² values. The two methods of soil chemical analysis yielded similar R² values for nitrogen uptake in plots both with and without nitrogen fertilization. The N mineral method was least useful in predicting sugar yield. Addition of the covariables rooting depth and mineralization capacity appreciably improved the explanatory value of the models with 0.01 M CaCl₂, especially when the analytical results of soil samples taken in autumn were used. For the N mineral method the addition of covariates was found to have far less influence.